i explored mod New civil war (StarRover) too, here author included besides others model Nova-battleship, but just little part of model (3kB imd), the big part (650 kB is missing).
Observing mechanism of model (small part is invissible) i got some ideas for rocket turret.

using common (THC1) as carrier for rocket modul.

burst is set to 0
range is set ala rocket

barrel´s imd can be hidden or replaced with rocket launcher imd, just optical effect, turret is still able firing shells

into barrel/rocket imd will be insert point for rocket modul, this way rocket modul will follow movement of turret´s gun barrel.

Real problem starts, when we want control it from user menu, because engine consider it like pure gun tower. How to "tell" to turret´s gun barrel, that there it has allowed rocket modul too.
Unfortunately, i am not good scripter, so content of the next lines is behind my practical knowledges.

Within mothership menu could by created extra window/frame (like kind of next fighter), where will be insert icon of rocket turret and this will activate extra window for rocket modul of rocket turret too.
it could probably demand changes in files .xsd (directory XMLSchema) and others too.

you buy rocket turret, place it and rockets will be reloaded through new window as usualy
or may be buy full and sell empty rocket turret, if reload will not work.
or icon of empty rocket tower will be deleted by script

if it will be impossible, there is still possibility create load/unload functions by script. I was read on StarRover forum, there boys plans run mod Retrospection on engine of sw2-ciwil war. There was comment, they have functional script for teleportation particular modules from mothership into fighter via script and hot key.
Similar way could be probably solved reloading of rocket modul conected into gun turret.
i know, it is just theory, but may be someone more experienced can confirm or refuse this idea.

I think the first right step would be to see if the game launches a rocket at all if you attached it to a turret, and then if it works, one can take on the GUI Problems of it, but I think changing how the game utilizes the GUI is impossible because it would require some changes which aren't possible with the .scr and etc files in GUI\, The engine itself would have to be modified in order for it to load new xmlschema parameters.

Quote:

they have functional script for teleportation particular modules from mothership into fighter via script and hot key.

Thats sounds quite interesting, could you tell me the thread where you read that?

EDIT: Nice to see they used some of my ships, including all of the eve ships (even the ones which arent finished) and the NOVA Battleship. Glad to see them in use for another mod.

Mounting other (rocket) imd into turret gun barrel is not problem, all attachments are displayed correctly, problem is activate rocket gunpoint via gui menu.
when you observe small guns, there is just small gun point inserted via imdeditor into imd of fighter, no imd of small gun with gunpoint is needed. i tried it on model of naginata, there you can see, that model looks the same with inserted gun or without it. I edited naginata imd, deleted all parts connected to smal gun, remains just little holes in body of fighter. With inserted small guns fighter was able shoting, no gun was visible, just fire and outgoing bullets.

And little from another theme, i explored model of Motherstone mamaship (STAS mod).
It looks like some joke, has double and triple parts in imd, textures look unusual too. So i decide made my own joke, Arba-stone mothership Need finish ref. points, gbnd, etc.
there is just screenshot.https://ulozto.cz/!G5sSsLhJMXHD/screenshots-zip

And little from another theme, i explored model of Motherstone mamaship (STAS mod).
It looks like some joke, has double and triple parts in imd, textures look unusual too. So i decide made my own joke, Arba-stone mothership Need finish ref. points, gbnd, etc.
there is just screenshot.

To be honest, it looks awesome, I like it. Even though its a joke.
Seems you're pretty good at merging ships together

It is nice, when i can see, that my creations enjoy others, but my amateurish metods i will better keep in secret.

to theme rocket turret.
Rocket point in fighter imd is enough to start fire rockets, though rockets are generated somewhere inside fighter. Easy can check, when use rocket launcher without valid gunpoint. Gunpoint in RL-imd just override starting place and can be use to adjust starting point of rocket more precisely.

So i finished serious joke arba-stone model. Two flies were killed by one shot.
This model is on theme Red Hammer too.
Guns and little bijouterie are ommited, there can be different opinions for it and it is no big problem add parts by own choice.